The asteroid, measuring 300 meters (984 feet) across and known as 2001 YB5, passed Earth at 7:37 a.m. For a moment, it was less than twice as far from Earth as is the Moon.

Scientists were unaware of its approach until a month ago, when it was spotted by an American observatory dedicated to tracking near-Earth objects (NEOs).

Astronomers insisted there was never any danger of a collision with Earth. But they warned that the asteroid was one of up to 400,000 small NEOs up to 1,000 meters (3,280 feet) wide that could strike Earth with little or no warning because of the absence of an adequate early-warning system.

The idea of a catastrophic asteroid strike has long been a source of morbid fascination, most recently manifesting itself in Hollywood disaster movies such as Armageddon and Deep Impact.

The potential consequences are indeed apocalyptic, according to research on asteroid impacts. If a 300-meter asteroid hit London, for example, it would destroy everything within a 95-mile (153-kilometers) radius and cause severe damage for a further 500 miles (804 kilometers), wiping out the United Kingdom, the Low Countries, and much of France.

In the more likely event of a similarly sized NEO landing in the sea (70 percent of Earth's surface is covered by water), it would trigger a series of tsunamis, or massive waves, that would devastate coastal regions.

Jonathan Tate, director of Spaceguard UK, said YB5 is one of hundreds of thousands of objects that present an unknown risk to the planet. Spaceguard UK campaigns for a British-funded telescope to watch for such asteroids as part of a global network.

"What limited resources exist for tracking asteroids are dedicated to spotting the 700 to 1,200 which are more than a kilometer in diameter and, if they hit the Earth, could wipe out the planet," Tate said. "That leaves very few resources for trying to trace the many more asteroids between 100 meters and 1,000 meters in diameter, which still present a very significant risk."

Tate added: "This particular asteroid passed us by without any danger, but it remains that by the time it was spotted last month, there was nothing we could have done to avert a catastrophe had it been heading towards us."